Indirectly heated cathode



Nov. 11, 1958 J. c. DURAN ET AL INDIRECTLY HEATED CATHODE Filed Jan. 3d,1956 MOUBIS INVENTOR JOHANNES CHRISTAAN DURAN JOZEPHUS FRANCISCUS DAMSGERARD CHARLES ANTONIE MARIE A ENT United States Patent INDIRECTLYHEATED CATHODE Johannes Christiaan Duran, Jozephus Franciscus Dams, andGerard Charles Antoine Marie Moubis, Eindhoven, Netherlands, assignors,by mesne assignments, to North American Philips Company, Inc., New York,N. Y., a corporation of Delaware Application January 30, 1956, SerialNo. 562,341

Claims priority, application Netherlands February 9, 1955 3 Claims. (Cl.313-346) The invention relates to indirectly heated cathodes for use inelectric discharge tubes, and more particularly to cathodes of this kindin which the space between the heater element and the cathode sleeve isfilled by sintered insulating material.

Frequently it is desirablethat the space available between the heaterelement and the sleeve of an indirectly heated cathode is entirelyfilled with insulating material, which will be sintered in position. Asexamples we may mention cathodes having a high operating temperature orcathodes in which a very high electrical insulation between the heaterelement and the cathode is required. In addition, it is frequentlydesirable to avoid relative movements of the heater element and thecathode in view of microphonic disturbances owing to variations in.capacity between the said parts.

It has, however, been found that it is very difficult to prevent theformation of cavities in the insulating material or between thismaterial and the cathode sleeve during the process of filling the spaceand after sintering. Sintering may be required in order to degas theinsulating material and to prevent said material from falling out of thecathode sleeve.

The formation of such cavities is undesirable, particularly in view ofthe heat transfer between the heater and the cathode sleeve.

That is, since the thermal conductivity is reduced by such cavities, andsince the cavities will be formed at different points in difierentcathodes, cathodes having difien ent operating temperatures will result,which non-uniformity is undesirable. Further, the heater may becomeoverheated locally so that the heater filament becomes brittle.

The said disadvantages can be entirely avoided by the use of a method ofmaking the cathode in which the insulating material is introduced intothe said space by sedimentation from a suspension.

This method provides the advantage of ensuring a partial separationbetween the finer and coarser particles of the suspension, particularlyif the height of fall, i. e., the distance the particles have to travel,is chosen sufficiently large. The coarser particles become locatedsubstantially at the. bottom and the finer particles substantially atthe top.

When the cathode sleeve is closed at one end, the greater part of thecoarser particles become consequently located in the proximity of theclosed end while the greater part of the finer particles become locatedin the proximity of the open end. The coarser particles are onlyslightly sintered and consequently shrink slightly only, while the finerparticles subsequent to sintering exhibit a great mechanical strengthand satisfactorily seal the open end. The coarser particles produce aporous mass having some resiliency so that the formation ofshrink-cavities is avoided.

A height of fall of from 2 to 3 times the length of the space to befilled has proved advantageous particularly 2,860,275 Patented Nov. 11,1958 when sedimentation is accelerated by the use of a centrifuge.

In order that the invention may readily be put into practice, oneembodiment will now be described in detail with reference to theaccompanying drawing, in which- Fig. 1 shows a holder for the cathodesleeve and the suspension during sedimentation, and

hFig. 2 shows a centrifuge provided with such holders, w ile Fig. 3 is asectional view of a cathode of the invention after sedimentation andsintering.

In Fig. 1, reference numeral 1 denotes: the emissive part of thecathode, in the case shown a dispenser cathode of the type described inU. S. Patents 2,543,728, 2,700,118 and 2,700,000, reference numeral 2the cathode sleeve or support in which the emissive part 1 is securedand supported, and 3 the heater arranged in the cathode sleeve 2. Thecathode sleeve 2 has a ridge 4 formed in its periphery. The assemblycomprising the cathode 1, the sleeve 2 and the heater 3 is introducedinto a metal holder 5 until the ridge 4 engages with the rim of theholder 5.v Said holder 5 has a cavity 9 formed in it which is providedwith a tapered part which registers with a second cavity within whichthe cathode sleeve fits. The holder 5 is arranged together with thecathode assembly in an aperture 7 of a second holder 6 which may, forexample, be made of synthetic resin. The cavity 7 has a recess 8 in itsbottom in which the cathode 1 is accommodated. The space 9 is now filledwith the suspension of the insulating material to be introduced into thecathode sleeve. Preferably use is made of ground and purified A1 0 1 kg.of ground A1 0 being suspended in 1 litre of ethyl alcohol. The size ofthe particles is preferably from 1 to 15 microns. The height of thespace 9 is at least equal to the length of the cathode sleeve 2. Thecoarser particles of the suspension settle first and consequently cometo rest substantially at the bottom in the proximity of the closed endof the cathode sleeve, the mean size of the particles decreasing in thedirection from bottom to top. When the solid substance is deposited, theliquid is decanted or evaporated and the holder 5 is taken from theholder 6 and heated until sintering of the insulating material occurs.In the cathode finally produced, which is shown in Fig. 3, the fillingof the cathode sleeve consists substantially of coarser particles 12(from 10 to 15 microns) while the percentage of finer particles 13(approximately 1 micron) gradually increases towards the open end.

The sedimentation can be considerably accelerated by the use of acentrifuge, as is shown in Fig. 2. The holder 6 can be suspended from anannular member 11 which is supported from arms 10 of a centrifuge. Witha height of fall of from 2 to 3 times the length of the cathode,substantially complete sedimentation is produced after a period of from5 to 10 minutes at a speed of 3000 revolutions per minute and with adistance between the cathodes and the axis of approximately 10 to 15cms., the solid substance filling the cathode envelope to the levelshown by a broken line 14 in Fig. l.

The sintering of the A1 0 is eifected by heating for two hours at 1000C. and subsequently for 15 minutes at 1600 C. It was found that nocavities are formed either during filling or during sintering. It isnecessary that the suspension contains particles of different sizes sothat preferably ground insulating material is to be used as the initialmaterial.

Although one embodiment of a cathode has been described, that is to saya so-called dispenser cathode, the invention can also be used withtubular cathodes.

What is claimed is:

1. An indirectly-heated cathode comprising a sleeve member and a heaterwithin said sleeve, and pulverulent in and closing ofi one end of saidhollow support member, 10

a heater element within said hollow support member, and a sintered' massof pulverulent insulating material filling the space within the hollowsupport member and surrounding the heater element, the portion of saidinsulating mass adjacent the electron-emitting member compri's- 15 4 ingmainly coarser particles, the portion of said insulating mass adjacentthe other end of said hollow support member comprising mainly finerparticles.

3. An indirectly-heated cathode as set forth in claim 1 wherein thepulverulent insulating material is ground aluminum oxide.

References Cited in the file of this patent UNITED STATES PATENTS2,647,216 Brown July- 28, 1-953 FOREIGN PATENTS 38l,551 Great BritainSept. 29,1932

117,857 Australia Dec. 2, 1 943

